When passive, UFOs patrol back and forth at a steady altitude, but will rise or descend to position themselves above the Spelunker if they find him.
Once directly above the Spelunker, they will fire a bolt of plasma downward which will deal 3 hitpoints of damage or destroy any block of terrain it hits. They do not need a line of sight, and will attempt to blast through obstructions to get to the Spelunker, similar to the enemies in Space Invaders.

Although this attack is extremely powerful, it is not the most dangerous factor to consider when fighting UFOs. When damaged, the UFO will fall straight down and crash-land. The second it hits any solid object or character, it explodes with the same force as a bomb. Being caught in the explosion is fatal, but even if you're clear of the initial blast, the explosion may set off a chain reaction with other UFOs, wipe out critical ledges in the already-cavernous Ice Caves, or propel rocks and other objects long distances.

UFO-related destruction also presents an enormous hazard if there are any Altars to Kali in the level, as they may fire upon you whilst sacrificing Yetis, destroying the altar and sabotaging your favor.

In some cases, the one-eyed Alien will bail out of a saucer that has been shot down, floating to the ground with a parachute. Once they touch the ground, they behave exactly like Snakes.

The safest method of wrecking UFOs is to use some manner of projectile, preferably while they are over a deep hole where they can fall and do no harm. Guns are usually ineffective, as the UFO will try to fly up out of your range.
Thrown objects such as rocks tend to be more successful, sometimes causing the ship to explode in mid-air.
However, be mindful that the explosion may cause the rock fly back at you, possibly causing damage. A better alternative can be a skull, pot or something that doesn't cause rebound damage to the player.

Of course, it is possible to whip or jump on top of a saucer to send it crashing to the ground, but it can be very difficult to avoid the subsequent explosion, depending on the nearby terrain.

Exploding UFOs can be used to your advantage if the ship is near a place that needs to be cleared. If you have no projectiles available you can throw a bomb, which will usually cause both the saucer and the bomb to explode mid-air.

The lasers shot by saucers can also be helpful if you can bait it to destroy certain blocks, allowing you to free some treasure without using bombs or climb into or out of certain places. They can also be used to easily kill enemies by knocking them down when they're above other enemies such as Yetis.